1. Field of the Invention
The disclosure relates generally to apparatus and techniques for mitigating signal reflections for signals in a data processing system, and more specifically relates to a connector and associated connector usage that mitigates signal reflections by eliminating stubs in a signal path.
2. Description of the Related Art
As processor speeds increase, there is a growing need to make improvements in the card and connector interface that connect to a plurality of cards and connects. As but one example, dual inline memory modules (DIMM) are plugged into various DIMM connectors on a system or motherboard to increase the amount of memory that is usable in a data processing system. The DIMM connectors are typically connected in a serial fashion on the system or motherboard, and introduce reflection-points or stubs in the electrical path or bus. FIG. 1A shows a traditional DIMM connector 100 without a DIMM card inserted within a cavity 115 of such DIMM connector. This DIMM connector has a plurality of attachment points 102 and 104 (only two are shown for ease in clarity) for connecting the DIMM connector 100 to a system or motherboard (not shown). The DIMM connector 100 also has a plurality of pins 106 and 108 (only two are shown for ease in clarity) for providing an electrical pathway from the attachment points 102 and 104 to a DIMM card when inserted in such DIMM connector. This connection can be seen in FIG. 1B that depicts a traditional DIMM connector 100 with a DIMM card 110 inserted therein. The pins 106 and 108 positively engage with wiring vias or connecting points (not shown) on the DIMM card 110, thereby providing an electrical connection from the DIMM card 110 to the system or motherboard by way of pins 106/108 and attachment points 102/104.
Certain connector assemblies for facilitating connection of a card or board inserted therein to a planar or motherboard also contemplate use of different lengths for the wires, strips, or wiring vias within the connector assembly to make it easier to insert and remove cards or boards. For example, as described in U.S. Pat. No. 4,095,866 entitled “High Density Printed Circuit Board and Edge Connector Assembly” which is hereby incorporated by reference as background material, two different strip lengths—a long strip length and a short strip length—are used to provide an electrical connection from the connector assembling to a card/board inserted into such connector assembly, as shown by elements 101/103 (long pins) and 105/107 (short pins) in FIG. 1C, that are part of the connector assembly 109 that is attached to a system or motherboard 111. The greater length of the spring contact members/strips reduces the force required for insertion of a card/board 113 into a cavity 115 within the connector assembly 109.
A depiction of signal paths within the system or motherboard interconnected to a DIMM card is generally shown at 200 of FIG. 2. Here, there are two DIMM connectors 100, specifically Connector 1 and Connector 2, with one (Connector 1) having a DIMM card 110 inserted therein, and the other (Connector 2) not having a DIMM card inserted therein. Only two representative bus signals 202 and 204 are shown for clarity, although in practice there are many bus signals including both address and data signals. The bus signals provide an electrical path between the system or motherboard and the memory devices. For example, Sig 1 element 202 provides an electrical path from the system/motherboard to Connector 1 through attachment point 102 and pin 106. Similarly, Sig 2 element 204 provides an electrical path from the system/motherboard to Connector 1 through attachment point 104 and pin 108. Connector 2 is arranged in a serial fashion with respect to Connector 1, and therefore Sig 1 element 202 also extends to element 102 of Connector 2 and Sig 2 element 204 also extends to element 104 of Connector 2 to provide an electrical path between the system/motherboard and Connector 2 in the event that a DIMM card were to be plugged into Connector 2 to increase memory capacity for the data processing system. Additional connectors can also be provided in this serial arrangement, and in such a case the Sig 1 element 202 and Sig 2 element 204 (as well as all other bus signals) would extend to such other connectors as indicated by the dotted lines for Sig 1 and Sig 2.
FIG. 3 provides a conceptual wiring view of series connections among multiple DIMM connectors on a system/motherboard at 300, where connector pads on the system/motherboard are shown and each path includes short or long traces and vias of the system/motherboard that provide the bus nodes to each of the connectors (not shown) mounted on the system/motherboard. When all of the connectors are not populated with a DIMM card, these wiring traces and vias present stubs 302 that act as reflection points, where the electrical signal that is activated to access a given DIMM card continues to travel along wiring traces and vias to its end, and then reflects back along the same wiring path back to the signal's originating point, as is known in the art. Current solutions to this stub-reflection problem include either providing some type of impedance-terminator at the end of the stub to absorb the electrical signal at the end of the stub, or to use the very end connector as the first DIMM card that is connected/plugged-in to reduce the length of the stub.
FIG. 4 shows at 400 a traditional DIMM connector with the numerous pins that support the numerous bus signals used to electrical interconnect with a DIMM card (not shown) when such DIMM card is inserted into the DIMM connector by using of a latching mechanism 402.
As shown above and summarized in FIG. 5, a problem exists when all card connectors are not populated with cards due to undesirable stub-reflections that adversely impact the maximum operational speed of the bus, thus negatively impacting overall system performance of a data processing system. Also note in FIG. 5 that when only one DIMM is populated in position C3 shown in the far-end configuration, stubs caused by the presence of the connector fingers of C1 and C2 will still adversely effect system performance. This technique is typically used if only one DIMM were used. However, this places the DIMM further away from the driver/receiver circuitry and forces the longest signal path and thus degrades system performance due to the longer path.